1. Field of the Invention
The present invention relates to an imaging device and a focusing control method, and in particular to an imaging device and focusing control method that perform focus control during imaging of a subject.
2. Related Art
Recently, accompanying increasing resolution of solid state image pick-up devices such as Charge Coupled Device (CCD) area sensors and Complementary Metal Oxide Semiconductor (CMOS) image sensors, there is a rapid increase in demand for information equipment with imaging functions, such as digital electronic still cameras, digital video cameras, mobile phones, and Personal Digital Assistants (PDAs, mobile data terminals). Information equipment with imaging functions such as those described above will be referred to in general as imaging devices.
Focusing control methods for detecting the distance to a main subject include contrast methods and phase difference Auto Focus (AF) methods. Phase difference AF methods are often employed in various imaging devices due to being able to detect the in-focus position faster and at higher precision than with contrast methods.
A rolling shutter method in which sequential resetting and reading is performed from the upper sensors is known as a reading method used in imaging devices equipped with CMOS sensors. Since time differences arise in read timing according to the pixel position in a rolling shutter method, distortion may arise in an image of a subject in cases of moving subjects.
Therefore, in a case in which a phase difference AF method is used for focus control in an imaging devices equipped with CMOS sensors when imaging a moving subject, there is an influence from distortion caused by the rolling shutter method, and errors may arise in phase difference detection due to image movements or image changes that occur during shifted read timings.
Japanese Patent Application Laid-Open (JP-A) No. 2009-128579 discloses a device that, in cases in which a reliable focusing detection result has not been obtained by focus detection pixels disposed in a horizontal direction, performs focus detection with focus detection pixels disposed in a vertical direction, and in cases in which movement of the subject is detected, prevents the focus detection with the focus detection pixels disposed in the vertical direction.
JP-A No. 2008-72470 and JP-A No. 2008-263352 disclose devices that control the charge accumulation timing for pixels for phase difference detection to be the same time.
However, the technology disclosed in JP-A No. 2009-128579 is merely technology in which focus detection is only performed using a phase difference AF method under limited conditions, such as according to the reliability of a focus detection result or cases in which movement of the subject has not been detected, and is not capable of reducing the influence caused by a rolling shutter method performed during focus detection by phase difference AF method. Moreover, since the technology disclosed in JP-A No. 2008-72470 and JP-A No. 2008-263352 requires an additional circuit, it increases cost.